Estimations of extreme wave conditions and corresponding maximal structural responses are of particular importance for the sake of ensuring structural safety of ships. The present paper is concerned with developing a method to predict the extreme value of structural response at the double bottom structure accounting for the double bottom bending moment by using FORM and the coupled CFD and FEA. The FORM and so-called Model-correction-factor approaches are adopted. The nonlinear strip method is used for predicting a design point (predictor) under the designated limit state function, then the CFD and FEA coupling method is used for correcting the predicted design point (corrector). Two types of limit state functions are used in estimating longitudinal stress level at the outer bottom surface in order to investigate the double bottom bending effect. The discussion is made in terms of identified critical wave episodes (design irregular waves) and associated structural responses. It turned out that the presented approach is valid for identifying the design irregular wave and corresponding reliability index where the structural response of the double bottom structure takes an extreme value.